This invention relates to a method for preventing solder from rising to a portion of an electric contact adapted to contact a mating object in producing the electric contact formed on a copper foil so as to extend therefrom by connecting the electric contact to the copper foil by soldering with enough bonding strength, and more particularly to an electric contact produced by the use of the method.
The word “rising” or “rise” as used herein is to be understood to mean a phenomenon that in soldering process, melted solder climbs up with the aid of the capillary action to an area where existence of the solder is unfavorable.
In order to bring a circuit board into contact with an electronic part, hitherto, if contact portions of the one part are flat, contact portions of the other part are frequently formed in extending shapes (for example, a hemisphere). As examples of such electric contacts, incorporated herein are Japanese Patent Application No. 2005-277,320 (Patent Literature 1) and Japanese Patent Application No. 2005-344,971 (Patent Literature 2) using metal balls, filed by the applicant of the present application.
Patent Literature 1
According to the abstract of the Japanese Patent Application No. 2005-277,320, the invention has an object to provide electric contacts and a method for producing the electric contacts having a predetermined height without causing defective connection between the electric contacts. Disclosed is an electric contact extending from a copper foil in which the copper foil is coated with a metal paste layer, and a metal ball is fixed to the copper foil by sintering the metal paste layer and is plated with gold over at least part adapted to contact a mating object. Further, disclosed is a method for producing an electric contact extending from a copper foil including steps of coating the copper foil with a metal paste layer over a predetermined area as a first step, arranging a metal ball on the metal paste layer and thereafter pushing the metal ball against the copper foil as a second step, fixing the metal ball to the copper foil by sintering the metal paste layer at a predetermined temperature as a third step, and plating the metal ball with gold over at least part adapted to contact a mating object as a fourth step.
Patent Literature 2
According to the abstract of the Japanese Patent Application No. 2005-344,971, this invention has an object to provide electric contact and a method for producing the electric contacts which have a predetermined height without any risk of defective or failed connection between the electric contacts. Disclosed is an electric contact extending from a copper foil, comprising a metal ball fixed to the copper foil by sintering a metal paste layer coated on the copper foil, the metal ball having a contact portion adapted to contact a mating object and plated with gold. Further disclosed is a method for producing an electric contact extending from a copper foil, comprising steps of coating a metal paste layer of a predetermined area on a copper foil as a first step, loading a metal ball on the metal paste layer and then pushing the metal ball to the copper foil as a second step, fixing the metal ball to the copper foil by sintering the metal paste layer at a predetermined temperature as a third step, and plating at least the contact portion of the metal ball adapted to contact a mating object with a noble metal as a fourth step.
In recent years, with the miniaturization of electric and electronic appliances, electrical connectors have been miniaturized and their pitches have become narrower. It is desirable to produce electric contacts on a copper foil using the metal balls disclosed in the Patent Literatures 1 and 2, from the standpoint of the narrower pitches and reduced overall height. It is needed to fix a metal ball to a copper foil by the use of an electrically conductive adhesive in order to install the surface-treated metal ball as an electric contact on the copper foil. Copper paste, silver paste and solder paste are generally used as the electrically conductive adhesive, while the electric contact is required to have a bonding strength as a mechanical property. By comparison, when metal balls having a diameter of 300 μm are bonded to a copper foil, laterally pulling-out strengths are 80 to 130 grf for copper paste, 120 to 190 grf for silver paste, and 300 to 500 grf for solder (Sn3Ag0.5Cu) paste. The solder paste is the best as to the bonding strength.
Moreover, the electric contacts are required to have superior electrical conductivity and low contact resistance as electrical properties. Needless to say, the bonded portions of the electric contacts are also required to have electrical conductivity and low contact resistance as electrical properties. By comparison, resistances are 50 μΩcm for copper paste, 40 μΩcm for silver paste, and 11 μΩcm for solder paste. The solder paste is the best as to the resistance.
Furthermore, as to resistance to environmental conditions, all the copper paste, silver paste and solder paste used as conductive adhesives are likely to be oxidized or sulfurized so that they are not suitable for contact portions adapted to contact mating objects, and stable connections with low contact resistance could not be obtained.
Moreover, as to rising or spreading of these pastes due to capillary action when fixing a metal ball by heating, there are significant differences between the copper paste or silver paste from the solder paste. The copper paste or the silver paste is made by kneading fine metal powder into epoxy family resin and solidified by heating. When being solidified, they do not rise or spread beyond the coated or printed regions. In contrast herewith, the solder paste is made by kneading powder of tin (Sn)-based alloy into a flux as a reducing activator. Before the solder arrives at its melting temperature, the flux starts to melt and spread due to capillary action so that the metal surface to be bonded by soldering is activated, with the result that the metal surface arrives a temperature sufficient to melt the solder. The rise of the solder with the capillary action would spread to the range whose angle is less than the contact angle. In other words, with the solder paste, the solder will rise to the contact portion of an electric contact adapted to contact a mating object and the raised solder reacts with the ambient air so as to be oxidized, whereby a stable connection with low contact resistance could not be obtained.